syntax: update a deriving error message to use the new syntax

Author: erickt

src/libsyntax/ext/deriving/clone.rs

@@ -188,9 +188,7 @@ fn expand_deriving_clone_struct_method(cx: @ext_ctxt,
                 fields.push(field);
             }
             unnamed_field => {
-                cx.span_bug(span,
-                            ~"unnamed fields in \
-                              expand_deriving_clone_struct_method");
+                cx.span_bug(span, ~"unnamed fields in `deriving(Clone)`");
             }
         }
     }

src/libsyntax/ext/deriving/eq.rs

@@ -289,7 +289,7 @@ fn expand_deriving_eq_struct_method(cx: @ext_ctxt,
                                             &mut outer_expr);
             }
             unnamed_field => {
-                cx.span_unimpl(span, ~"unnamed fields with `deriving_eq`");
+                cx.span_unimpl(span, ~"unnamed fields with `deriving(Eq)`");
             }
         }
     }

src/libsyntax/ext/deriving/iter_bytes.rs

@@ -191,7 +191,7 @@ fn expand_deriving_iter_bytes_struct_method(cx: @ext_ctxt,
             }
             unnamed_field => {
                 cx.span_unimpl(span,
-                               ~"unnamed fields with `deriving_iter_bytes`");
+                               ~"unnamed fields with `deriving(IterBytes)`");
             }
         }
     }